1. Field of the Invention
This invention relates to flow control valves, and more particularly, to a flow control valve capable of accurate regulation at very low flow rates in systems which experience either widely varying inlet pressures or significant temperature variations.
2. Description of the Prior Art
There are systems in which it is desirable to have available a stream of gas or liquid flowing at a very low but constant flow rate. An example of such a system is a pneumatic system for measuring the depth of a liquid in a tank, such as has been used in the pulp and paper industry. In such a system air under pressure is delivered into a constant flow valve and the effluent of such valve is delivered into the body of liquid and discharged therein near the bottom of the tank. Variations in the depths of the liquid are reflected in changes of the back pressure experienced at the outlet of the air pipe. Since the flow through such pipe is constant, the change in the back pressure can be readily measured by a pressure gauge located in such pipe and such pressure gauge can be calibrated, if desired, to provide a direct reading corresponding to the change in the height of the liquid in the tank.
Known flow control valves have proven unsuccessful in such systems in which quite low constant flow rates are desired. This is at least partially due to a wide variation in pressure of the compressed air that is available through use of conventional economically practical means. One type of conventional flow control valves utilizes a pressure differential controlled variable area orifice in series with a fixed orifice or restriction. Another type employes a conventional automatic constant pressure regulator in series with a manually variable flow control orifice. Neither of these arrangements performs satisfactorily in a low flow system throughout the full range of pressures encountered.
Additionally, known flow control valves operating at low flow rates must often be recalibrated when subjected to significant temperature variations. Such valves often use an extremely small diameter orifice to obtain the desired low flow rate and hence any size deviation caused by thermal expansion or contraction will accordingly vary the flow rate through the control orifice. A related problem of known flow control valves capable of operating at low flow rates concerns the size of the restriction through which the regulated fluid must flow. A fluid supply source, typically a pneumatic system employing air, often contains small undesirable solid particles which will collect around a small control orifice and change the flow characteristics through the orifice. Accordingly, after an extended period of use this build up of foreign matter around the orifice restriction is often of sufficient magnitude to change the pre-calibrated flow rate.
Examples of known flow control valves which purportedly produce constant flow at low rates, are disclosed by U.S. Pat. No. 3,028,876, granted Apr. 10, 1962, to J. L. Gratzmuller, and by U.S. Pat. No. 1,699,676, granted Jan. 22, 1929, to Earl S. Rusch. However, it has been found that valves of this type do not actually produce constant flow within the desirable limits at flow rates of about 0.01 cubic feet per hour to about 2.0 cubic feet per hour when used with an economical source of compressed air.
Another type of known flow regulator for controlling ultra-low volumetric flow rates is disclosed by U.S. Pat. No. 3,749,113, granted July 31, 1973, to Jack Isreeli, Aaron Kassel, and Edwin H. Mernyk. This patent is of interest because it presents a very good discussion of the various problems encountered which make it difficult to accurately regulate flow at ultra-low volumetric flow rates. However, the valve proposed by this patent represents a different approach to solving the problem than I have taken.